Question

I am working on creating a class to handle very large numbers for use in Project Euler challenges. I have overloaded both << and + to work with the class. The following compiles and works properly:

BigNum a(400000000);
BigNum b(400000000);
BigNum c;

c = a + b;

cout << c; 

The following produces a compile error:

BigNum a(400000000);
BigNum b(400000000);

cout << (a + b); 

The error is: error: no match for ‘operator<<‘ in ‘std::cout << BigNum::operator+(const BigNum&) const(((const BigNum&)((const BigNum*)(& b))))’

For reference, here is the class definition and implementation:

/* BigNum.h */

#include <iostream>
#include <vector>

class BigNum {
public: 
BigNum();
BigNum(const BigNum &src);
BigNum(int num);
void print(std::ostream *os);

public:
BigNum &operator=(const BigNum &src);
BigNum &operator+=(const BigNum &src);
const BigNum operator+(const BigNum &src) const;

private:
bool negative;
std::vector<int> digits;
};

std::ostream &operator<<(std::ostream &os, BigNum &bNum);

/* BigNum.cpp */

#include "BigNum.h"

BigNum::BigNum() {
digits.push_back(0);
}

BigNum::BigNum(const BigNum &src) {
digits = src.digits;
}

BigNum::BigNum(int num) {
if(num == 0) {
    digits.push_back(0);
}
else {
    while(num != 0) {
        digits.push_back(num % 10);
        num /= 10;
    }
}
}

void BigNum::print(std::ostream *os) {
int i;

for(i = digits.size() - 1; i >= 0; i--) {
    *os << digits[i];
}
}

std::ostream &operator<<(std::ostream &os, BigNum &bNum) {
bNum.print(&os);
return os;
}

BigNum & BigNum::operator=(const BigNum &src) {
if(this == &src) return *this;

digits = src.digits;

return *this;
}

BigNum & BigNum::operator+=(const BigNum &src) {
int carry, i, k;

carry = 0;

if(digits.size() < src.digits.size()) {
    digits.resize(src.digits.size(), 0);
}

{
    k = src.digits.size();
    for(i = 0; i < k; i++) {
        digits[i] += src.digits[i]; 
        digits[i] += carry;
        carry = 0;
        if(digits[i] > 9) {
            carry = 1;
            digits[i] -= 10;
        }
    }

    k = digits.size();
    for(i = src.digits.size(); i < k; i++) {
        digits[i] += carry;
        carry = 0;
        if(digits[i] > 9) {
            carry = 1;
            digits[i] -= 10;
        }
    }

    if(carry) {
        digits.resize(digits.size() + 1, 1);
    }
}

return *this;
}

const BigNum BigNum::operator+(const BigNum &src) const {
BigNum result(*this);
result += src;
return result;
}

Why does

cout << (a + b);

not work? There are obvious work-arounds, but I want to understand why this is happening. I am new to C++, so if there is anything else I am doing incorrectly or could be doing better, please let me know. Thank you.

Answer 1

Your overload of operator<< doesn’t take the const BigNum that operator+ returns. Change the parameter type in operator<< to const BigNum &.

The function thinks it’s allowed to modify the object passed in, but you’re passing a non-modifiable object, so it doesn’t match.

As pointed out below, the print() function you call from there needs to be const as well.